[Illustration: FIG. 33.—Development of the egg of the common oyster, after fertilisation within the tubular passage of the reproductive sacs. A, surface view. B, section through a very early stage—the separate cells or protoplasmic corpuscles which have resulted from the dividing up of the fertilised single egg-cell are seen; _bl_, in-pushing to form the gut; _sk_, in-pushing to form the rudiment of the double shell. C and D, the same a few hours later. The mouth, _m_, is now seen. E, still later stage surface view: a ring of cilia has appeared. F, the young free-swimming oyster nearly ready to leave its mother’s protection, who is now laden with such young, and is said to be “white-sick.” The top of the head, _tp_, is now well marked and surrounded by a ring of lashing cilia. The outline of the right-side shell is seen, and the foot, _ft_, between the mouth, _m_, and the anus, _a_. The stomach, _st_, and the intestine, _e_, show by transparency.] In the course of a week or so the brood of dark young oysters escapes by thousands from the parent’s shell into the surrounding water. They swim by their circlet of vibrating hairs, or “velum,” as it is called, towards the surface, and are carried far and wide by the tides. They are active, transparent little “dots,” very unlike their parent (Fig. 34). The next thing that happens to them—after a few days, perhaps weeks—is that owing to the increasing weight of their shells, they sink to the bottom. More than half perish by dropping thus on to bad ground; a vast number have already been eaten by young fishes and shrimps. Those which are lucky enough to fall on to something hard—stones, rocks, old oyster-shells, or the shells of living oysters—become cemented to those hard substances by the new shelly substance formed by the growing edge of the lowermost of their little shells, which now spread out, lose their cockle-like shape as they grow, and become, the one (the left by which it is fixed) large, deep, and bossed, the other flat. The little oysters are only one-fortieth of an inch in diameter when first they become fixed, but they grow rapidly, feeding in the same way as their parents. Vast numbers are eaten by other animals. In some localities in two years, in others in three years, they have grown to a couple of inches in length, and now produce in the summer breeding season a certain quantity of eggs and sperm to start new generations. The oyster continues to grow, and at five to seven years of age is in full vigour and maturity; at ten years of age it produces few eggs, or sperm-cells; and in the course of another year or so, under natural conditions, dies. [Illustration: FIG. 34.—Free-swimming young oyster or oyster-larva, showing the head, with its tuft of cilia projecting from between the two shells, _l_ and _r_.]